运行一个.NET应用程序或者运行库宿主时，OS会首先建立一个进程，然后会在进程中加载CLR(这个加载一般是通过调用_CorExeMain或者_CorBindToRuntimeEx方法来实现)，在加载CLR时会创建一个默认的AppDomain，它是CLR的运行单元，程序的Main方法就是在这里执行，这个默认的AppDomain是唯一且不能被卸载的，当该进程消灭时，默认AppDomain才会随之消失。

-To run a. NET runtime host application, or when, OS will first establish a process, then loaded in the process CLR (this is usually loaded by calling _CorExeMain or _CorBindToRuntimeEx method to achieve), when loading the CLR creates a default AppDomain, it is the CLR running unit, the program' s Main method is where the implementation of the default AppDomain is the only and can not be unloaded, and when the process of elimination, the default AppDomain will disappear.